Judd Apatow – king of the tangent, lord of the wander – bumps a subplot up to full story status with this directionless sequel-cum-spin-off to his 2007 hit Knocked Up.

It's five years since ambitious TV journalist Alison (Katherine Heigl) got pregnant by a shabby stoner (Seth Rogen) after a drunken one-night stand. Mum, dad and baby are doing so well that there's no need to visit them here.

Instead, we'll spend 134 minutes with Alison's sister, Debbie (Leslie Mann, Apatow's off-screen wife) and her husband Pete (Paul Rudd). They live in an affluent suburb of LA with their two kids, Sadie and Charlotte (Apatow's kids, Maude and Iris). Debbie owns a clothes shop, Pete runs a failing record label. They're both approaching their 40th birthday, and have decided to celebrate by ploughing headlong into a crisis.

Pete and Debbie aren't sure they love each other any more. They hide their bad habits (cupcakes for Pete, cigarettes for Debbie). They avoid each other. He's evasive, she's a nag. At the same time, though, they love each other: because of the kids, because of history, because of responsibility. They're perched on life's midpoint, looking down the hill and realising that what they have right now is what they'll have forever.

The couple haven't developed as characters since Knocked Up. Pete's still bumbling, a little careless. Debbie's still panicky and shrill. This worked fine when they were a footnote to somebody else's story but, given their own film, they're really hard to care about. Their financial problems – as a result of Pete's handouts to his luckless dad (Albert Brooks) and a foolhardy plan to relaunch the career of 70s rocker Graham Parker – are supposed to form the backbone of the plot. Instead you find yourself sneering at these rich people, crowing at the suggestion that downgrading a big house for a smaller big house is a problem worthy of our sympathy.

Crucially, This Is 40 isn't funny. Part of the problem is the script, which works hard to wring laughs out of bone-dry sitcom staples (the moody teen, the foul-mouthed toddler, the sex-starved married couple), but Mann and the kids are unfortunately at fault too. Mann has a habit? Of pausing in the middle of the punchline? As if you should be savouring? The joke? And the kids – likable enough, but by no means great actors – feel stranded. Maude's character is little more than a Lost-obsessed, iPad-clutching misery guts. Iris (bar one charming piece of physical comedy where she acts out a cartoon death) is adrift: left to recite lines fed to her by her dad, whether she gets them or not. "If this is what having a period is like, I don't want one!", she intones after Maude has stomped out of the room again. You almost expect a Cosby Show laugh track to drop in and seal the scene.

In interviews, Apatow comes across as pleasant, thoughtful and surprisingly grounded. As a mentor, he's had an immeasurable effect on American comedy, shepherding the best comics of recent years to popular attention (a number of them – Lena Dunham, Chris O'Dowd, Melissa McCarthy, Jason Segel – return the favour here with neat, if under-developed, bit parts). But, without anything to laugh at, think on or care about, this is just a scrapbook of semi-interesting anecdotes, a loosely edited sprawl. It's not bad, exactly – but it is boring and very rarely funny. This is laboured. This is aimless. This Is 40. It's really quite a grind.